Flow Control For Geothermal Well

ABSTRACT

Systems and methods for harvesting geothermal energy use temperature-based flow control to optimize the extraction of thermal energy from a geothermal reservoir. In one example, a thermal transport fluid is flowed into a wellbore traversing a thermal reservoir of a formation. Flow of the thermal transport fluid into and out of the thermal reservoir is dynamically controlled at each of a plurality of injection and/or return locations in response to a downhole parameter such as temperature. For example, flow may be controlled so that the flow into the thermal reservoir is greater at the injection locations where the temperature is hotter and that the flow out of the thermal reservoir is greater at the return locations where the temperature is hotter. The thermal transport fluid produced from the return locations is then conveyed to surface to extra the thermal energy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/488,907, filed Sep. 29, 2021, which is incorporated by reference inits entirety.

BACKGROUND

The energy industry continually strives to supplement existing,well-developed energy resources and technologies, such as by developingemerging energy sources and discovering new energy sources. Thesesupplemental forms of energy may be useful, both to provide redundantenergy and power system in the short term and to increase energyavailability and reliability in the longer-term. So-called “alternative”energy sources, including those that are not so economically viabletoday, may also be phased in over a long time scale as other resourcesgradually deplete.

One area of robust development is in geothermal energy production.Geothermal energy production is a way to harvest existing energy in theform of heat inside the earth (geothermal energy) and converting thatthermal energy into other forms of useful energy. Geothermal energyproduction systems have been constructed in an effort to extract thermalenergy below ground and convert it to electricity.

Globally, it has been speculated that significant levels of geothermalpower can be generated using closed-loop geothermal systems that exploithot and deep geothermal resources. While these systems can becommercialized in some locations presently, the industry generally seeksimprovements in well construction technology to broaden the range ofprojects that can be more economically viable. Such improvements mayinclude, for example, efforts to reduce the cost of drilling, improvingmaterials, and advancing well completion systems used to harvestgeothermal energy.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings illustrate certain aspects of some of the embodiments ofthe present disclosure and should not be used to limit or define themethod.

FIG. 1 is an elevation view of a geothermal energy production systemaccording to a first example configuration having a separate injectionwell and return well.

FIG. 2 is a side view of a more specific example configuration of thesystem generally shown in FIG. 1 , with a particular arrangement ofvalves and optional zonal isolation.

FIG. 3 is a side view of an example of a thermostatic control valve(TCV) that mechanically and autonomously controls flow in relation totemperature.

FIG. 4 is a side of the TCV, wherein the sleeve has been shifted by anexpansion chamber from a closed position to an open position in responseto a temperature increase.

FIG. 5 is a side view of the TCV with an optional side pocket mandrelfor housing controller components.

FIG. 6 is a schematic diagram of a system having a controller forcontrolling a plurality of valves in relation to temperature.

FIG. 7 is a schematic diagram of another example system providingtemperature-based flow control at a plurality of locations within eachof a plurality of zones.

FIG. 8 is an elevation view of a geothermal energy production systemaccording to another example configuration, wherein the injection andreturn locations are along the same well.

DETAILED DESCRIPTION

Disclosed herein are systems and methods for harvesting geothermalenergy from an underground geothermal reservoir (alternately referred toherein as simply “thermal reservoir”). Aspects include optimizing flowbetween the thermal reservoir and surface to maximize energy extractionat a surface of a well site. The disclosed principles may be applied toany of a wide variety of thermal energy extraction. By way ofillustration, example embodiments below discuss thermoelectric powergeneration, such as by using a thermoelectric generator. However, energymay be extracted using any of a variety of systems and for a variety ofpurposes. Other examples of extracting and utilizing thermal energyinclude heating a building or a green house, extraction of “processheat” used in industrial processes, such as manufacturing processeswhich require mixing of chemicals at elevated temperatures, andco-generation. The extracted energy may be useful across a broadspectrum of industries, including food and beverages, textiles,machinery industries, chemical industries, as well as agriculture anddomestic/household type uses.

The disclosed systems and methods may control flow to and/or from aplurality of injection and return locations along a reservoir inrelation to a downhole parameter of the thermal transport fluid, such astemperature, pressure, volume, heat, flow, velocity, isothermal changes,isobaric changes, isochoric changes, phase change, acoustic, vibration,distributed sensing (distributed temperature sensing (DTS), distributedacoustic sensing (DTS), distributed pressure sensing (DPS), and otherparameters that may be measured via extrinsic sensors and/or intrinsicsensors, etc.) or pH, as non-limiting examples. In some embodiments,these systems and methods may implement temperature-based flow controlto and from a thermal reservoir to improve the recovery of thermalenergy, such as to optimize efficiency or rate of energy production.Temperature-based flow control may be based on temperature measurements.Where temperature sensors are used, the temperature sensors may besufficiently spaced from the injection or return locations (e.g., awayfrom the valves) so as to not to introduce an adverse feedback loopanalogous to an air-conditioning unit blower blowing on the thermostat.Temperature-based flow control may alternatively rely, at least in part,on other parameters related to temperature, such as pressure and volumeare related to temperature according to the Ideal Gas Law. In oneexample, the systems and methods may ensure a thermal transport fluid(gas, liquid, or combination of both) is injected into and flows outfrom hotter portions of the geothermal zone occupied by the thermalreservoir to improve efficiency. The flow of the thermal transport fluidbetween surface and the thermal reservoir may be along one or morewells, of which various example configurations are shown. The flow maybe facilitated by one or more pumps. Alternatively, flow may befacilitated by thermosiphoning, which is based on natural convection tocirculate a fluid without the necessity of a mechanical pump.

In some examples, the system and method may involve the use of injectionvalves for controlling the flow of thermal transport fluid into thethermal reservoir and a plurality of return valves for controlling theflow of the thermal transport fluid out of the thermal reservoir. Thevalves can be, for example, inflow control valves (ICVs) and/orthermostatic control valves (TCVs). In some configurations, the valvesmay be autonomous to provide automatic, temperature-based flow control,optionally using mechanical means. In other examples, a controllercomprising a central controller and/or a plurality of local controllersmay be in communication with temperature sensors to control flow at theinjection and return valves in relation to temperature. In any of theseexamples flow may be individually, dynamically adjusted at eachinjection and return valve to ensure that flow is greater to hotterinjection and locations than to corresponding cooler injection andreturn locations.

In other examples, the flows may be adjusted and/or regulated based uponother parameters than injecting and/or returning fluid into and/or fromthe hottest locations. For example, it may be desirable for thetemperature of the return fluid to be lowered, hence a flow from acombination of higher and lower temperature zones may be combined toprovide the desired return fluid temperature at the surface. In otherexamples, the TCV may incorporate pressure sensors, or other sensors(pH, sonic, e.g., fiber optic sonic or sensing fiber, flow sensors, masssensors, mass-flow rate sensors, venturi, seismic, pH, fiber-opticsensors (intrinsic sensors and/or extrinsic sensors), etc.

FIG. 1 is an elevation view of a geothermal energy production system 10according to a first example configuration having a separate injectionwell 20 and return well 40. The system 10 is constructed at a site of anunderground formation 12 having any number of earthen formation layersor strata, e.g., 12 a, 12 b, and an underground thermal reservoir 14deep below a surface 16 of the earth. The thermal reservoir 14 maycomprise a region wherein temperatures are very hot and where the strata12 b is relatively fluid permeable. The permeability of the formation 12may allow a large amount of a thermal transport fluid 18 to be stored inthe thermal reservoir 14, that is continually heated by the surroundingformation 12 in the vicinity of the thermal reservoir 14. The system 10is used to circulate heated thermal transport fluid 18 from the thermalreservoir 14 to the surface 16 for extracting thermal energy from thethermal transport fluid before reinjecting the thermal transport fluidinto the geothermal reservoir 14. For discussion purposes, the energyextraction in these examples involves generating electricity, as part ofa thermo-electric power generating cycle. However, capabilities of thesystem 10 may also include other flow and thermal control such asthermal recovery, and may be applied to any form of energy extraction.

Generally, a thermal transport fluid is a fluid (e.g., a gas or liquid)that participates in heat transfer by serving as an intermediary inheating on one side of a process, transporting and storing thermalenergy, and cooling on another side of the process. In the example ofFIG. 1 , the thermal transport fluid 18 is heated in the vicinity of thethermal reservoir 14, then transports the heated fluid to the surface,where the heat is extracted for energy conversion. Water is a commonheat transfer fluid because of its economy, high heat capacity andfavorable transport properties. For higher temperatures, oil orsynthetic hydrocarbon or silicone based fluids offer lower vaporpressure. Gases such as water vapor, nitrogen, argon, helium andhydrogen have been used as heat transfer fluids where liquids are notsuitable. Supercritical fluids such as carbon dioxide are also suitableas a heat transfer fluid. For example, supercritical CO2 can providelarge compressibility and expansivity, which can facilitate athermosiphon, whereby the thermal transport fluid is circulated withless or no reliance on mechanical pumping.

The thermal reservoir 14 may be hot, dry rock with appreciably no nativefluids; or, the thermal reservoir 14 may have an existing, naturallyoccurring reserve of fluids, such as water and/or hydrocarbon fluidslike oil and gas, which permeate the formation 12. Typically, however,the size and/or capacity of the thermal reservoir 14 may be increased byinjecting additional, non-native fluid supplied from the surface 16(e.g. water or supercritical CO2) over a period of time (e.g., days orweeks) until there is a sufficient volume of thermal transport fluid 18that is sufficiently heated by the formation 12 to economically drivethermoelectric power generation.

In some cases, a system may be defined wherein the thermal reservoir isnot exclusively comprised only of naturally-occurring geologicalreservoir elements. A thermal reservoir may include a volume in thevicinity of a heat-containing geological formation having a capacity tostore thermal energy. For example, a thermal reservoir may be defined toinclude a tube (e.g., a casing used to reinforce a wellbore) that mayaccept and hold heat and transfer heat. In another embodiment, the areabetween the casing and wellbore may be an element of a thermalreservoir. In a related embodiment, a thermal reservoir may include acasing-wellbore annulus and the volume/area inside the casing may beconsidered a thermal reservoir. A thermal reservoir may be a fixedvolume or a volume/area that changes. A thermal reservoir may haveboundaries that change. For example, a thermal reservoir may consists ofone set of boundaries and then a change in boundaries may change toincrease/decrease the size, shape, or other characteristic of thethermal reservoir. As another example, a thermal reservoir may consistof the area between two isolation packers with a permeability of 10millidarcy and porosity of 12%. In another instance the thermalreservoir may change due to additional pressure being applied to forcethe thermal transport fluid out into a larger volume of the formation(rock, etc.). Thus, the thermal reservoir may be any of a variety ofconfigurations of a space capable of storing thermal energy and alsochanging temperature by energy-transfer to (or from) another body(conduction) or fluid (convention) or radiation, or any combinationthereof. Also thermal transfer due to other sources) such as heat fromthe core of the earth, etc.

The system 10 in this example includes a separate injection well 20 andreturn well 40 drilled into the formation down to the thermal reservoir14. The system also includes an electric generator 60 installed at thesurface 16 of the site. The injection well 20, return well 40, andgenerator 60 may form a closed loop fluid path for circulating thethermal transport fluid 18 as part of a thermo-electric power generatingcycle. The thermal transport fluid 18 is continually flowed from thethermal reservoir 14 to the surface 16 via the return well 40 to haveits thermal energy converted to electricity by the generator 60. Thethermal transport fluid 18 is cooled by virtue of extracting the thermalenergy. The cooled thermal transport fluid 18 is then reinjected intothe thermal reservoir 14 via the injection well 20 to be reheated. Thisthermoelectric power cycle continues indefinitely, drawing thermalenergy from the earth and converting it to another form of energy suchas electricity at the surface 16.

The injection well 20 and return well 40 may each be created by drillinginto the formation 12 using any suitable drilling technique known in theart. Directional drilling techniques may be used to guide the path ofeach of the injection well 20 and the return well 40 from a verticalsection extending from the surface 16 to a respective angular orhorizontal section that traverses a length of the thermal reservoir 14.The depth of the injection well 20 may be selected to reach a levelwhere there is sufficient heat in the formation 12 to make successful,cost effective thermal production practical. The undergroundtemperatures effective for geothermal energy systems may range from, forexample, 150 to 500 degrees Celsius. The drilling depths are generallyin the range from 1,000 ft to about 30,000 ft. The types of formationssuitable for geothermal energy production may vary depending on thechoice of thermal transport fluid. For example, a system usingsupercritical CO2 may benefit from a permeability in a different rangethan a fluid like water. The higher the permeability in the rockformation the easier the fluid will flow but the more fluid can be lostinto the formation 12.

The generator 60 may convert thermal energy of the thermal transportfluid 18 into electrical energy using any of a variety of methods knownin the art apart from the specific teachings of this disclosure. Forexample, the thermal transport fluid 18 may be water or supercriticalCO2 that is converted to steam and used to power rotation of a turbine62. Alternatively, the thermal transport fluid 18 may be flowed througha heat exchanger to heat a secondary working fluid in a separate loopthat is used to power rotation of the turbine 62. Examples of secondaryworking fluids include, but are not limited to, pentane, isobutane, ahalogenated hydrocarbon refrigerant, liquid ammonia or another suitableRankine-cycle working fluid. Kinetic energy of the rotating turbine 62may then be converted to electricity by a converter unit 64. Forexample, the converter unit 64 may comprise a rotor and a stator thatuse the rotation of the turbine 62 to produce relative motion between amagnet and a conductor to generate an electromotive force (EMF)according to Faraday's law of induction. The power generated from theturbine generator 60 may be used on site or transferred to a powerplant. The electricity can then be injected into an electrical grid forcommercial use, for example.

A number of variables may influence the efficiency of the thermoelectricpower generation. In this and other disclosed embodiments, the system 10may help improve or optimize efficiency of the thermoelectric powercycle by dynamically controlling the flow of thermal transport fluid 18to and from each of a plurality of injection locations and returnlocations in relation to temperature. The system includes a number ofinjection valves 22 along the injection well 20 at respective injectionlocations and return valves 42 along the return well 40 at respectivereturn locations. In FIG. 1 five injection valves 22 and four returnvalves 42 are schematically shown by way of example, but any number ofvalves may be used. Temperature may also be sensed, directly orindirectly and by electronic, mechanical, fluid, or other means, at eachof a plurality of temperature locations 24 along the injection well 20and temperature locations 44 along the return well 40. Flow may then beindividually controlled at each valve 22, 42 in relation to temperature.For example, each valve 22, 42 may incorporate a temperature responsiveelement to control flow or be in communication with a discretetemperature sensor to control flow. The system 10 may dynamicallycontrol flow at each valve 22, 42 in relation to temperature to provideincreased flow to hotter regions and cooler flow to cooler regions aboutthe thermal reservoir 14. That is, the valves control the flow inrelation to temperatures proximate to the temperature responsiveelements such that the flow of thermal transport fluid into the thermalreservoir is typically greatest at the hottest injection locations andflow from the thermal reservoir back into the wellbore is typicallygreatest at the hottest return locations.

The temperature locations 24, 44 may correspond to locations of discretetemperature sensors along the formation or wellbore. Alternatively, thetemperature locations 24, 44 may correspond to locations where afiberoptic temperature sensor senses temperature along the formation orwellbore. In other cases, such as when using a thermostatic controlvalve (TCV), the temperature locations 24, 44 may correspond to thelocations of the respective TCVs themselves. using a temperature sensorto sense a temperature proximate each of the plurality of injectionlocations and return locations; and dynamically adjusting the flow atthe respective injection locations and return locations in relation tothe sensed temperature proximate the respective injection locations andreturn locations. In some installations it is beneficial to sensetemperatures at temperature locations that are spaced far enough fromthe respective valves so that the flow of hot steam or other flowphenomenon do not adversely affect the temperature readings, introducehysteresis issues, or other potential sources of error. The appropriatespacing may be very system-dependent. For example, in some embodiments aspacing on the order of between 50 and 150 feet apart from each of therespective injection locations and return locations may be far enoughaway to avoid such effects, yet close enough to give a representativeindication of temperature at the respective valve. Typically, even whentemperatures are sensed at a distance away from the valves, thetemperature associated with a given valve is taken at a location that iscloser to that valve than to other valves in the system. Other,non-distributed sensors may be utilized separately or in conjunctionwith fiber-optic based distributed temperature sensing (DTS) and/ordistributed acoustic sensing (DAS).

In some embodiments, obtaining a temperature gradient in the vicinity ofa valve may enhance temperature-based flow control. A temperaturegradient may be obtained in some examples using two or more temperaturesensors associated with each valve in order to determine a temperaturegradient within one or more zones. For example, in one or moreembodiments, three (or more) sensors per valve may be desirable in orderto determine a thermal gradient in both directions from the valve, e.g.,uphole towards the surface and downhole towards the toe (end) of thewellbore. Again, in other embodiments, the temperature gradient may besensed using DTS and/or DAS. As compared with taking a single-pointtemperature reading at a single, discrete temperature sensor location,the use of multiple spaced-apart temperature sensors and/or distributedsensing to obtain a temperature gradient may provide a better indicationof temperature behavior.

A number of downhole valve types are generally known in the art apartfrom the specific teachings of this disclosure, any of which may be usedaccording to the disclosure to control flow of a thermal transport fluidfrom a wellbore into a thermal reservoir and from a thermal reservoirback into the wellbore. For example, a number of ICVs are available thatgenerally have the capability of controlling flow between a formationand a wellbore. A number of TCVs are also generally know in the arthaving the capability of controlling flow between a formation and awellbore. The general capabilities of these valves of controlling flowinto and out of a wellbore may be used in the particular manner andconfiguration described in specific examples below, and variationsthereof, all of which are considered within the scope of thisdisclosure. Thus, the systems and methods of this disclosure are notlimited to the specific example configurations and valve types shownbelow.

FIG. 2 is a side view of a more specific example configuration of thesystem 10 generally shown in FIG. 1 including a particular arrangementof valves and optional zonal isolation. Generally, the injection well 20and return well 40 may be spaced apart or otherwise arranged so that thethermal transport fluid injected into the thermal reservoir 14 at theinjection well 20 will traverse a portion of the thermal reservoir 14and be sufficiently heated before flowing out of the thermal reservoir14 back into the return well 40. In the example depicted in FIG. 2 , thereturn well 40 is directly vertically above the injection well 20, butother relative arrangements are possible in keeping with the principlesof this disclosure. For example, wellbores at an angle from the verticalthat allow the wellbore to intersect natural fractures in the thermalreservoir. In other instances, one, or both, of the wellbores may runparallel to the bed of the reservoir. For example if the reservoirbed/formation is inclined 10-degrees from horizontal, the one or morewell bores may also be inclined 10-degrees from the horizontal. If thereservoir has a dipping bed (on a curved trajectory), the one or morewellbores may be positioned accordingly to maximize the thermalrecovery, provide the most economical well bore to drill, and/or withother objectives in mind.

A set of injection valves 22 and return valves 42, individuallyreferenced as 22 a-c and 42 a-c, are installed in the respectiveinjection and return wellbores 20, 40. The injection valves 22 a-c andreturn valves 42 a-c are preferably interconnected in respective tubularstrings 26, 46 which are installed in respective slotted, screened orperforated liners 28, 48 positioned in open hole portions of therespective wellbores 20, 40. Three injection valves 22 a-c and threereturn valves 42 a-c are depicted in each wellbore in FIG. 2 by way ofexample. However, any number of valves may be used in keeping with theprinciples of the invention so long as there are multiple injectionvalves and/or return valves spaced apart so that temperature along therespective wellbores 20, 40 can be differentiated and flow controlledaccordingly. The injection valves 22 a-c and return valves 42 a-c may beany of the valves 12 described herein, or other suitable valves known inthe art apart from the specific teachings of this disclosure.

A plurality of zones 15 a-c of the formation 12 may be isolated fromeach other in an annulus 70 between the perforated liner 48 and thereturn well 40, and in an annulus 72 between the perforated liner 28 andthe injection well 20, using a sealing material 74 placed in eachannulus. The sealing material 74 could be any type of sealing material(such as swellable elastomer, hardenable cement, selective pluggingmaterial, etc.), or more conventional packers could be used in place ofthe sealing material. The zones 15 a-c may also be isolated from eachother in an annulus 76 between the tubular string 46 and the liner 48,and in an annulus 78 between the tubular string 26 and the liner 28, bypackers 80 or another sealing material. This zonal isolation is notrequired in every configuration but may be included where helpful tocontrol flow into selected zones. The zones 15 a-c may also be isolatedfrom each other in the formation by injection of a sealing or barriermaterial. Methods of injection plane initiation in a well may beutilized to place the sealing and/or barrier material into the preferredarea.

In the well system 10, thermal transport fluid is injected into thethermal reservoir 14 at zones 15 a-c of the formation 12 via therespective injection valves 22 a-c in the injection well 20, andformation fluid (including the injected fluid) is received from thezones into the respective return valves 42 a-c in the return well 40.Thermal transport fluid injected into the zones 15 a-c is represented inFIG. 2 by respective arrows 21 a-c, and thermal transport fluid producedfrom the zones is represented in FIG. 2 by respective arrows 23 a-c.

The injection valves 22 a-c and return valves 42 a-c in the wellbores20, 40 are used to control flow of thermal transport fluid into and outof the formation 12 at the respective injection and return locations. Atemperature variation across the thermal reservoir 14, which may includeone or more temperature taken in each zone or more particularly attemperature locations in proximity to injection and return locations maybe used to individually control flow into and out of each zone. Bycontrolling the flow into and out of the zones 15 a-c according totemperature, an efficiency may be optimized.

For example, flow of the thermal transport fluid injected into a coolerzone may be choked or completely shut off, while flow to a hotter zonemay be increased. As temperatures change over time, the flow at eachinjection and return location may be dynamically adjusted in response totemperature so that flow into and out of the thermal reservoir 14 islarger at hotter zone(s) than at cooler zones.

In one example, each injection valve 22 a-22 c and/or each return valve42 a-c may be a thermostatic control valve (TCV) that may individuallyregulate flow in response to temperature by mechanical means. As anotherexample, a temperature variation along the wellbores 20, 40 may bemonitored using individual temperature sensors, or a fiber optic line 52in the injection well 20 and/or a fiber optic line 50 in the productionwell 40. Each fiber optic line 50, 52 may be queried at differentlocations along its length to determine a temperature at severallocations. Signals from the temperature sensors and/or or fiber opticlines may be input to a control module of each valve, so that each valveappropriately adjusts its associated flow relative to the other valves.In some embodiments, a control module may control one or more valves. Insome embodiments, one or more control modules may be placed at thesurface. In one or more embodiments the control modules may be downhole,uphole, near the wells and/or at a location far from the wells.

FIG. 3 is a side view of an example of a thermostatic control valve(TCV) 120 suitable for use with this disclosure that opens and closesmechanically and autonomously in relation to temperature. The TCV 120 isan example of a temperature-responsive valve for dynamically controllingthe flow of the thermal transport fluid in relation to a temperature atthe respective injection locations and/or return locations. The TCV 120in this example regulates flow in relation to temperature usingmechanical means and can be configured for use as either an injectionvalve or as a return valve. The TCV 120 is shown in a closed state inFIG. 3 . The TCV valve comprises a valve body 122 with a valve body port124 (which may be a plurality of circumferentially spaced ports) forflow to or from an annulus about the valve body 122. The valve body 122may be fluidically coupled in line with a tubing string via a threadedconnection 125 or the like. A sleeve 126 is disposed within the valvebody 122 and is axially moveable within the valve body 122 to open orclose the valve body port 124. The sleeve 126 also include a sleeve port128 that may be aligned or misaligned with the valve body port 124 toopen and close the TCV 120 respectively. In FIG. 3 , the sleeve port 128is misaligned to the right of the valve body port 124, so that flow isclosed between the annulus about the TCV 120 and the interior of the TCV120. A mechanical expansion chamber 130 (which may comprise multipleexpansion chambers arranged in series) is coupled to the sleeve 126. Theexpansion chamber 130 expands or contract in response to increasing ordecreasing temperature to urge the sleeve 126 to alternately open andclose the valve body port 124. With the TCV 120 closed, flow isconstrained to flow through the valve body 122 without entering orexiting the valve body port 124.

Again, valve-selection may be discretionary based on the type of welland the operator preferences. In some operations, for example, anoperator may elect to go with TCVs only on the production side of theoperation. In other operations, TCVs may be elected to be used only onthe injection side of operations. In other embodiments, TCVs may placesin only one local (zones) on the injection and/or production side of thewell or wells. For example, one or more TCVs may be placed in only themost permeable zones of the one or more wells. In other embodiments, itmay be preferred to use one or more TCVs to regulate only the flow intoa geothermal well, a geothermal zone. In other embodiments, one or moreTCVs may be used in one of a series of multilateral wells, of one ormore wellbores in a geotherrmal system of wellbores.

FIG. 4 is a side of the TCV 120 of FIG. 3 , wherein the sleeve 126 hasbeen shifted to an open position in response to a temperature increase.In particular, an expansion chamber 130 has increased in volume inresponse to the temperature increase, urging the sleeve 126 to theright, such that the sleeve port 128 now aligns with the valve body port124. The thermal transport fluid flows out through the aligned ports124, 128, as in the example of using the TCV as an injection valve.Alternatively, such a TCV could be configured to allow a fluid to beinjected into a formation when the ports are so aligned. As illustratedby example in FIGS. 3 and 4 , such a TCV is optionally capable ofcontrolling flow in relation to temperature using mechanical means andwithout necessarily having any electronic input. Alternatively, however,a valve with a sliding sleeve mechanism or other such mechanism could beconfigured to be controlled with an electronic controller that wouldsense temperature and control position of the sleeve in response totemperature.

FIG. 5 is a side view of the TCV 120 of FIGS. 3 and 4 configured with anoptional side pocket mandrel. The side pocket mandrel comprises amandrel 132 (which may be part of a valve body) with a side pocket 134for holding a control unit 136 for operating the TCV 120. The controlunit 136 may comprise the mechanism or a portion thereof, e.g., thesleeve itself and/or the expansion chambers, that controls the positionof the sleeve or other moveable valve element within the TCV 120 inresponse to temperature. Alternatively, another type of valve may beconfigured so that the side pocket 134 may house an electronic modulefor controlling the opening and closing of that valve. Positioning thecontrol unit 136 in the side pocket 134 provides for accessibility tothe control unit 136 and other components. In other embodiments, thecontrol unit 136 and/or sleeve port 128 may be made retrievable viawireline, coiled tubing or other systems/methods. This way control unit,136 and sleeve port 128, for example, can be replaced which isadvantageous because they are the components which are more likely tofail due to wear, corrosion, temperature degradation, erosion, etc. Byutilizing a side pocket mandrel, components such as control unit, 136and sleeve port 128, seals, sealing devices, (not shown) between sleeveport 128 and valve body port 124 may be replaced without withdrawing thetubing string, such as tubular strings 26, 46, from a well.

Further, the control unit 136 can be retrieved from a wellbore forrepairs, maintenance, replacement, and/or a set point change. Thecontrol unit may be removed and replaced with another control unit, ifdesired, by lowering a tool deployed on wireline or coiled tubing intothe wellbore through the delivery conduit. A tool shaped to fit aroundthe end of the control unit could be used to hold the control unit as itis being retrieved from or lowered into the wellbore. Fiber optic cablecould be placed along the wellbore on the outside of the fluid (e.g.,steam) line to detect the temperature along the entire length of thewellbore. Alternatively, temperature sensors could be placed along thewellbore. Such temperature monitoring methods could be used to indicatewhen a brain needs to be replaced with another one having different setpoints.

The foregoing discussion provides some examples of system configurationsand elements thereof, including examples of valves and other systemcomponents suitable for controlling flow into or out of a thermalreservoir responsive to temperature. These and other systems maycomprise mechanical or electromechanical components. Generalized controlsystems and control logic will now be discussed in relation to FIGS. 6and 7 that may be applied to any of a variety of system embodimentsusing mechanical or electromechanical control functionality.

FIG. 6 is a schematic diagram of a system 20 having a controller 210 forcontrolling a plurality of valves 221-225 in relation to temperature. Itshould be recognized, for temperature control, the Combined Gas Law mayapply, which describes a relationship between temperature, pressure andvolume. Therefore, it is understood when discussing the controlling oftemperature this may entail not only the control of temperaturedirectly, but may also take into consideration the control of pressureand volume. Since volume of a thermal reservoir may be variable, thecontrol of pressure and temperature is especially inter-related withinthe scope of this disclosure. The controller 210 may include a centralcontroller located at surface or downhole in communication with theplurality of valves 221-225. Alternatively, a distributed controller maycomprise individual controllers 230 included with each valve 221-225, inaddition or in lieu of the central controller 210. Such individualcontrollers 230 could operate autonomously in relation to localtemperatures sensed. Alternatively, such individual controllers 230could cooperate with the central controller 210 to implement a controllogic for controlling the valves in relation to temperature.

In any configuration, the valves 221-225 are spaced apart along awellbore 212 with sufficient spacing that temperature may varyappreciably along a portion of the wellbore 212 spanning the valves221-225. A temperature T1 through T5 is indicated for each of therespective valves 221-225. A temperature profile 214 is superimposed onthe figure to graphically indicate how temperature varies from valve tovalve 221-225. In this example, at a moment in time, temperature T3 inproximity to the third valve 223 is the hottest temperature andtemperature T5 in proximity to the fifth valve 225 is the coolesttemperature. The flow Q1 through Q5 is individually adjusted at eachvalve according to temperature. Thus, for example, the flow Q3 at thethird valve 223 may be highest flow of the five valves 221-225, and theflow Q5 at the fifth valve 225 may be the lowest of the five valves221-225. In some cases, one or more valves at the lower end of thetemperature profile 214 may have their flow on a minimum setting, whichmay be completely shut off, while valves at the upper end of thetemperature profile 214 may have their flow at or near a maximumsetting. The valves that are neither at the highest or lowesttemperature locations may have their flow settings individually adjustedsomewhere at or above the flow of the lowest flow setting (Q5) and at orbelow the flow of the highest flow setting (Q3).

In some cases, the valves 221-225 may be individually adjustable to oneof a plurality of discrete flow settings such as fully on, fully off,and optionally one or more flow settings between fully on and fully off.In other examples, each valve may have an “infinite” adjustabilitybetween fully on and fully off. In cases where the valves have multipleor infinite adjustability, the controller 210 may control flow withsufficient granularity that each valve may have a different flow rate atany given point in time based on there being a different temperatureproximate every valve. In other cases, where a valve is only switchablebetween fully on and fully off, then the valves may just be switched onor off depending on whether they are toward an upper end of thetemperature profile 214 (ON) or toward a lower end of the temperatureprofile 214 (OFF).

In some cases, the valves 221-225 may be autonomous, and individuallycontrol their own flow rates responsive to temperature withoutcommunication with the other valves and without knowing whattemperatures are in proximity to the other valves. For example, amechanical TCV such as discussed in relation to FIGS. 3 and 4 above maybe pre-configured to turn on or off or adjust flow responsive totemperature based on mechanical expansion of a chamber, withoutexplicitly measuring a temperature. In that case, each valve 221-225 maybe preconfigured to open/close or vary a flow rate in response totemperature so that any given valve in a given temperature range willrespond similarly, and so that valves above a certain temperature willhave greater flow than valves below that temperature.

Alternatively, each valve may have a local controller 230 that eithersenses temperature locally or is in electronic communication with anoptional discrete temperature sensor or fiber optic temperature sensor,to electronically adjust flow rate in response to temperature. Again,this can be an autonomous flow control, without necessarilycommunicating with other valves or with a central controller 210. Inthat case, the individual controllers 230 of each valve 221-225 may bepreconfigured to open/close or vary a flow rate in response totemperature so that any given valve in a given temperature range willrespond similarly, and so that valves above a certain temperature willhave greater flow than valves below that temperature.

Yet another alternative uses the central controller 210, remotelylocated from the valves (e.g., at surface or somewhere else downhole) toimplement the control logic. In that case, the central controller 210may be in communication with the optional discrete temperature sensors232 or a fiber optic continuous temperature sensor to obtain thetemperatures T1-T5, and dynamically adjust the flow rates of therespective valves 221-225, so that flow generally increases withincreasing temperature. In some cases, flow may be individuallycontrolled at each valve with sufficient granularity that each valve isat a different flow rate than other valves at different temperatures. Inother cases, even if the flow control is less granular, any given valveat a higher temperature is at the same or greater flow rate than anothervalve at a lower temperature. Exceptions may occur temporarily, such asif the temperature profile 214 changes and there is a lag in adjustingthe various valves, which may result in a temporary situation where avalve at a hotter location is at a lower flow rate than a valve at acooler location.

Another optional feature is to normally use the controller 210 toautomatically generate command signals responsive to the temperatureproximate each of a plurality of injection locations and returnlocations for individually controlling the flow of the thermal transportfluid at the respective injection locations and return locations. Then,the controller 210 and command signals can be selectively overriddenwith manual control signals, such as at a surface of a wellsite, toallow an operator to instead manually control the flow at the respectiveinjection locations and return locations.

In some embodiments, the valves may be manually adjusted such as bytripping a wireline or other conveyance downhole to manually adjust adesired valve, or to replace the valve or change out components tochange the flow rate, in relation to temperatures sensed at the surface.Such embodiments may be useful with installations where the temperaturesare relative static and changes are infrequent.

Thus, a method of producing geothermal energy may include flowing athermal transport fluid into a wellbore traversing a thermal reservoirof a formation. A flow of the thermal transport fluid from the wellboreinto the thermal reservoir may be dynamically controlled at each of aplurality of injection locations along the wellbore such that the flowis greater at the injection locations where the temperature is hotter. Aflow of the thermal transport fluid from the thermal reservoir into thewellbore may also be dynamically controlled at each of a plurality ofreturn locations along the wellbore such that the flow is greater at thereturn locations where the temperature is hotter. The thermal transportfluid from the return locations may then be provided to an electricgenerator to generate electric energy. In any of the foregoing examples,flow is generally increased to hotter injection locations and returnlocations (and, correspondingly, decreased to cooler injection andreturn locations). By directing more flow of thermal transport fluidinto and out of the hottest locations of the formation, the efficiencyof the thermoelectric cycle may be optimized.

FIG. 7 is a schematic diagram of another example system providingtemperature-based flow control at a plurality of locations within eachof a plurality of zones along the wellbore. In this example, theinjection well 20 is organized into two zones 320, and the return well40 is organized into three zones 340, although any number of zones 320and/or 340 could be created. The zones are pressure-isolated from oneanother with a suitable sealing element 310 such as a plug or packer oneach side of each zone, including between adjacent zones. Each zonecomprises two or more of the injection locations where injection valves322 are positioned and two or more return locations where return valves342 are positioned.

Each zone may be managed independently of other zones, to individuallycontrol flow through the valves in that zone in relation to temperatureproximate the injection and return locations. For example, each zone maybe operated according to the general principles of the system 200 ofFIG. 6 . Within each zone 320 along the injection well 20, flow isindividually controlled from the wellbore 20 into a thermal reservoir ateach of the injection locations corresponding to the injection valves322 in that zone, such that the flow is greater at the injectionlocations where the temperature is hotter within that zone 320.Likewise, within each zone 340 along the return well 40, the flow fromthe thermal reservoir into the wellbore at each of the return locationscorresponding to the return valves 342 in that zone is controlled suchthat the flow is greater at the return locations where the temperatureis hotter within that zone 340.

In some of the foregoing examples, a separate injection well and returnwell are provided. The injection and return wells may be entirelyseparate wells in some examples. In other examples, the injection andreturn wells may be separate branches of a multilateral well.

FIG. 8 is an elevation view of a geothermal energy production system 10according to another example configuration, wherein the injection andreturn locations are along the same well 420. The plurality of returnlocations where return valves 442 are positioned are along the samewellbore as the injection locations where the injection valves 422 arepositioned. Thus, a first wellbore portion spanning the plurality ofinjection locations is coextensive with a second wellbore portionspanning the plurality of return locations (rather than in separatewells or separate wellbore branches).

To achieve this, the well 420 includes separate injection and returnflowpaths isolated from one another. In particular, an injection conduit424 is disposed along the wellbore in fluid communication with theplurality of injection valves 422. The injection conduit may be an innertubing string, for instance. A return conduit 444 is also provide alongthe wellbore in fluid communication with the plurality of return valves442. The return conduit 444 may be, for instance an annulus between thewellbore and the injection conduit 424. An insulation is providedbetween the injection conduit 424 and the return conduit 444, so thatcooler fluid being injected into the well 420 via the injection conduit424 is insulated from the hotter fluid in the return conduit 444 that isused to drive thermoelectric power generation at the generator 460.

Those of ordinary skill in the art in possession of this disclosure willappreciate that other well configurations are also within the scope ofthis disclosure aside from the example wellbore arrangements and flowpaths discussed herein. In some embodiments the hotter fluid in a returnconduit may be used for other purposes beside thermoelectric powergeneration. As examples, the hotter fluid may be used for heating abuilding, a green house, or for manufacturing processes which requireheat such as mixing of chemicals at elevated temperatures, etc. Further,in some embodiments, other components or areas of the well may beinsulated to reduce heat loss and increase thermal efficiency, etc. Insome embodiments, the portion of the well above the geothermal reservoirmay be insulated so the heat is not transferred out of well and into thesurrounding wellbore, formation, water reservoirs, etc.

Accordingly, the present disclosure may provide flow control into andout of a geothermal reservoir for extraction of energy at surface. Anon-limiting example application of such flow optimization includesthermoelectric power generation that optimizes efficiency and/or poweroutput by virtue of controlling flow in response to temperature ortemperature related parameters. The methods/systems/compositions/toolsmay include any of the various features disclosed herein, including oneor more of the following statements.

Statement 1. A method, comprising: flowing a thermal transport fluidfrom a surface location to a geothermal reservoir along an injectionflow path of a wellbore; flowing the thermal transport fluid into thegeothermal reservoir at each of a plurality of injection locations alongthe injection flow path; flowing the thermal transport fluid from thegeothermal reservoir into a return flow path of the wellbore at each ofa plurality of return locations; flowing the thermal transport fluidalong the return flow path back to surface; extracting thermal energyfrom the thermal transport fluid from the return flow path beforeflowing the thermal transport fluid back into the injection flow path ofthe wellbore; and dynamically controlling flow into or out of thegeothermal reservoir in response to one or more downhole parameters ofthe thermal transport fluid.

Statement 2. The method of Statement 1, wherein extracting thermalenergy from the thermal transport fluid comprises using the thermalenergy to drive an electric generator.

Statement 3. The method of any of Statements 1 or 2, wherein dynamicallycontrolling flow into or out of the geothermal reservoir in response toone or more downhole parameters of the thermal transport fluid comprisesincreasing a flow rate of the thermal transport fluid to or from hotterlocations of the geothermal reservoir.

Statement 4. The method of Statement 3, further comprising: using valvesfor dynamically controlling the flow of the thermal transport fluid inrelation to the one or more downhole parameters at the respectiveinjection locations and/or return locations.

Statement 5. The method of any of Statements 1 to 4, further comprising:sensing one or more temperature along the wellbore to identify injectionlocations or return locations where temperature is hotter; anddynamically adjusting the flow at the respective injection locations orreturn locations in relation to the one or more downhole parameters.

Statement 6. The method of any of Statements 1 to 5, further comprisingsensing the one or more downhole parameters with a fiber-optic sensoralong a portion of the wellbore spanning one or both of the plurality ofinjection locations and the plurality of return locations.

Statement 7. The method of any of Statements 1 to 6, wherein theinjection flow path and the return flow path are along differentwellbores.

Statement 8. The method of any of Statements 1 to 7, wherein theinjection flow path and the return flow path are along the samewellbore.

Statement 9. The method of any of Statements 1 to 8, further comprising:automatically generating command signals, responsive to the one or moredownhole parameters proximate each of a plurality of injection locationsor return locations for individually controlling the flow of the thermaltransport fluid at the respective injection locations and returnlocations.

Statement 10. The method of Statement 9, further comprising: selectivelyoverriding the automatically generated command signals with manualcontrol signals at a surface of a wellsite to instead manually controlthe flow at the respective injection locations and return locations.

Statement 11. The method of any of Statements 1 to 10, furthercomprising: pressure-isolating a plurality of zones along the wellbore,wherein each zone comprises two or more of the injection locations ortwo or more of the return locations; within each zone, individuallycontrolling the flow of the thermal transport fluid from the wellboreinto the thermal reservoir at each of the injection locations in thatzone in response to the one or more downhole parameters of the thermaltransport fluid; and within each zone, individually controlling the flowof the thermal transport fluid from the thermal reservoir into thewellbore at each of the return locations in that zone in response to theone or more downhole parameters of the thermal transport fluid.

Statement 12. A geothermal energy extraction system, comprising: awellbore in fluid communication with a geothermal reservoir at aplurality of injection locations and/or return locations; a valve ateach injection location for controlling a flow of a thermal transportfluid from the wellbore into the geothermal reservoir and/or a valve ateach return location for controlling a flow of the thermal transportfluid from the geothermal reservoir into the wellbore; and wherein thevalves control the flow in relation to one or more downhole parametersproximate to the valves.

Statement 13. The geothermal energy extraction system of Statement 12,wherein the valves control the flow of thermal transport fluid such thatthe flow is greater at hotter injection locations and at hotter returnlocations.

Statement 14. The geothermal energy extraction system of Statement 12 or13, further comprising: a power generator to generate power from thethermal transport fluid received from the return locations.

Statement 15. The geothermal energy production system of any ofStatements 11 to 14, wherein each valve comprises a port, a sleeveaxially moveable to open or close the port, and an expansion chambercoupled to the sleeve that expands in response to increasing temperatureto urge the sleeve to open the port.

Statement 16. The geothermal energy production system of any ofStatements 11 to 15, wherein each valve comprises a controller forregulating the flow of thermal transport fluid through the valve and aside pocket for holding the controller.

Statement 17. The geothermal energy production system of any ofStatements 11 to 16, further comprising: a sensing system for sensingthe one or more downhole parameters at a plurality of locations alongthe wellbore; and a controller in communication with the valves andsensing system and configured for adjusting each valve according to theone or more downhole parameters sensed at a nearest of the temperaturelocations.

Statement 18. The geothermal energy production system of Statement 17,wherein the controller comprises a local controller at each valve and acentral controller in communication with all of the local controllers.

Statement 19. The geothermal energy production system of Statement 17 or18, further comprising: a plurality of zones along the wellborepressure-isolated with sealing material, wherein each zone comprises aplurality of the injection locations and/or a plurality of the returnlocations; and wherein the controller is configured for adjusting thevalves in each zone in relation to temperatures in proximity to thevalves within that zone, such that the flow of thermal transport fluidis greater at some of the injection locations and at some of the hotterreturn locations within each zone.

Statement 20. The geothermal energy production system of any ofStatements 11 to 19, wherein the thermal transport fluid comprisessupercritical CO2.

To facilitate a better understanding of the present invention, thefollowing examples of certain aspects of some embodiments are given. Inno way should the following examples be read to limit, or define, theentire scope of the disclosure.

For the sake of brevity, only certain ranges are explicitly disclosedherein. However, ranges from any lower limit may be combined with anyupper limit to recite a range not explicitly recited, as well as, rangesfrom any lower limit may be combined with any other lower limit torecite a range not explicitly recited, in the same way, ranges from anyupper limit may be combined with any other upper limit to recite a rangenot explicitly recited. Additionally, whenever a numerical range with alower limit and an upper limit is disclosed, any number and any includedrange falling within the range are specifically disclosed. Inparticular, every range of values (of the form, “from about a to aboutb,” or, equivalently, “from approximately a to b,” or, equivalently,“from approximately a-b”) disclosed herein is to be understood to setforth every number and range encompassed within the broader range ofvalues even if not explicitly recited. Thus, every point or individualvalue may serve as its own lower or upper limit combined with any otherpoint or individual value or any other lower or upper limit, to recite arange not explicitly recited.

Therefore, the present embodiments are well adapted to attain the endsand advantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent embodiments may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Although individual embodiments arediscussed, all combinations of each embodiment are contemplated andcovered by the disclosure. Furthermore, no limitations are intended tothe details of construction or design herein shown, other than asdescribed in the claims below. Also, the terms in the claims have theirplain, ordinary meaning unless otherwise explicitly and clearly definedby the patentee. It is therefore evident that the particularillustrative embodiments disclosed above may be altered or modified andall such variations are considered within the scope and spirit of thepresent disclosure.

What is claimed is:
 1. A method, comprising: flowing a thermal transportfluid from a surface location to a geothermal reservoir along aninjection flow path of a wellbore; flowing the thermal transport fluidinto the geothermal reservoir at each of a plurality of injectionlocations along the injection flow path; flowing the thermal transportfluid from the geothermal reservoir into a return flow path of thewellbore at each of a plurality of return locations; flowing the thermaltransport fluid along the return flow path back to surface; extractingthermal energy from the thermal transport fluid from the return flowpath before flowing the thermal transport fluid back into the injectionflow path of the wellbore; and dynamically controlling a flow of thethermal transport fluid into or out of the geothermal reservoir inresponse to one or more downhole parameters of the thermal transportfluid and/or of the geothermal reservoir, comprising sensing the one ormore downhole parameters at a plurality of sensor locations along thewellbore, each sensor location spaced far enough from a respective oneof the injection locations or return locations that the flow does notdirectly affect the sensed downhole parameters, and dynamicallyadjusting the flow at the respective injection locations or returnlocations in relation to the sensed downhole parameters.
 2. The methodof claim 1, wherein the sensor locations are more than 150 feet awayfrom the respective injection locations or return locations.
 3. Themethod of claim 1, wherein the sensor locations are between 50 and 150feet away from the respective injection locations or return locations.4. The method of claim 1, wherein the sensor locations are less than 50feet away from the respective injection locations or return locations.5. The method of claim 1, wherein the one or more downhole parameterscomprise a plurality of temperatures.
 6. The method of claim 5, furthercomprising sensing one or more of the downhole parameters with afiber-optic sensor along a portion of the wellbore spanning one or bothof the plurality of injection locations and the plurality of returnlocations.
 7. The method of claim 5, further comprising sensing theplurality of temperatures using discrete temperature sensors.
 8. Themethod of claim 5, further comprising sensing the plurality oftemperatures using a combination of discrete sensors and distributedsensors.
 9. The method of claim 1, further comprising: automaticallygenerating command signals, responsive to the one or more downholeparameters proximate each of a plurality of injection locations orreturn locations for individually controlling the flow of the thermaltransport fluid at the respective injection locations and returnlocations
 10. The method of claim 9, further comprising: selectivelyoverriding the automatically generated command signals with manualcontrol signals at a surface of a wellsite to instead manually controlthe flow at the respective injection locations and return locations. 11.The method of claim 1, further comprising: pressure-isolating aplurality of zones along the wellbore, wherein each zone comprises twoor more of the injection locations or two or more of the returnlocations; within each zone, individually controlling the flow of thethermal transport fluid from the wellbore into the thermal reservoir ateach of the injection locations in that zone in response to the one ormore downhole parameters of the thermal transport fluid; and within eachzone, individually controlling the flow of the thermal transport fluidfrom the thermal reservoir into the wellbore at each of the returnlocations in that zone in response to the one or more downholeparameters of the thermal transport fluid.
 12. A geothermal energyextraction system, comprising: a wellbore in fluid communication with ageothermal reservoir at a plurality of injection locations and/or returnlocations; a valve at each injection location for controlling a flow ofa thermal transport fluid from the wellbore into the geothermalreservoir and/or a valve at each return location for controlling a flowof the thermal transport fluid from the geothermal reservoir into thewellbore; wherein the valves control the flow in relation to one or moredownhole parameters relative to the valves; a sensing system comprisingsensors for sensing the one or more downhole parameters at a pluralityof sensor locations along the wellbore, each sensor location spaced farenough from a respective one of the valves that the flow through thatvalve does not directly affect the sensed downhole parameter; acontroller for dynamically adjusting the flow at the respectiveinjection locations or return locations in relation to the senseddownhole parameters; and a power generator to generate power from thethermal transport fluid received from the return locations.
 13. Thegeothermal energy extraction system of claim 12, wherein the sensorlocations are more than 150 feet away from the respective injectionlocations or return locations.
 14. The geothermal energy extractionsystem of claim 12, wherein the sensor locations are between 50 and 150feet away from the respective injection locations or return locations.15. The geothermal energy extraction system of claim 12, wherein thesensor locations are less than 50 feet away from the respectiveinjection locations or return locations.
 16. The geothermal energyextraction system of claim 11, wherein the one or more downholeparameters comprise a plurality of temperatures.
 17. The geothermalenergy extraction system of claim 16, wherein the sensors comprisediscrete temperature sensors for sensing the plurality of temperatures.18. The geothermal energy extraction system of claim 16, wherein thesensors comprise one or more distributed temperature sensors for sensingthe plurality of temperatures.
 19. The geothermal energy extractionsystem of claim 16, wherein the sensors comprise a combination ofdiscrete and distributed sensors.
 20. The geothermal energy extractionsystem of claim 12, further comprising: a tubing string disposed alongthe wellbore, wherein an injection flow is defined along an interior ofthe tubing string in fluid communication with the plurality of injectionvalves and a return flow is defined along an annulus between thewellbore and the tubing string in fluid communication with the pluralityof return valves.